The present invention relates to new ceramics and their preparation method. More particularly, it relates to novel ceramics made of flaky .beta.-SiC containing .beta.-SiC as the major component; a process for preparation of such materials; and their applications. The objects of the present invention are to provide new ceramics with improved properties in resistance to thermal shock, thermal fatigue and oxidation which are prepared from an organic silicon polymer compound (to be explained later in detail) as the starting material; and to manufacture novel ceramics and refractories with desirable physico-chemical properties by utilizing the above ceramics as the starting material.
Flaky .beta.-SiC of the present invention (or to be described as thin fragments or scales) is a hitherto unknown type of ceramics and is obtained by following method in which thin sheet is prepared with an organic silicon polymer compound containing the carbon and silicon atoms as the major skeletal components, the sheet is made infusible by conventional methods; the infusible sheet is cut into flakes and finally the flakes are heat-treated in the atmosphere of a non-oxidative gas. This material finds utility in the starting material for new sintered ceramic compact and refractories having a unidirectional or random laminary structure. The organic silicon polymer employed in the present invention is a high-molecular organic silicon compound containing the silicon and carbon atoms as the major skeletal components which has been synthesized by Prof. Yajima et al., the Research Institute for Iron, Steel and Other Metals, Tohoku University. It is well known that SiC fibers derived from such organic silicon polymers have also become well known world-wide by many papers and patents filed by Prof. Yajima et al.
The organic silicon polymer has opened a new field of research in materials and is utilized as the starting material for fibers as well as for binders, impregnants and coating compounds.
The present inventors have succeeded in preparation of a new sheet or flaky .beta.-SiC from the organic silicon polymer and one of the major objects of the present invention is to provide special thermo-resistant ceramics having excellent resistance to thermal shock, thermal fatigue and oxidation by utilizing such flaky .beta.-SiC.
Fundamentally speaking, the organic silicon polymers employable in the present invention have the following unit structures: ##STR1## wherein R.sub.1 is --CH.sub.3 ; and R.sub.2, R.sub.3 and R.sub.4 are one or more members selected from the group consisting of hydrogen, alkyl, aryl, (CH.sub.3).sub.2 CH--, (C.sub.6 H.sub.5).sub.2 SiH-- and (CH.sub.3).sub.3 Si--.
k, l, m and n show the numbers of repetition of the unit structures defined by () and [], and usually vary in the following ranges: k=1-80; l=15-350; m=1-80; n=15-350. The average molecular weight of the organic silicon polymers is in the range of 800-20000.
In unit structure III, M is a metallic or nonmetallic element such as Si, B, Ti, Fe, Al, Zr, Cr and the like, and may be contained in the starting material and/or is mixed in the starting material during the use of the catalyst employed for synthesis of the organic silicon polymer and is contained in the major skeletals. R.sub.5, R.sub.6, R.sub.7 and R.sub.8 are one or more members selected from the group consisting of hydrogen, alkyl, aryl, (CH.sub.3).sub.2 CH--, (C.sub.6 H.sub.5)2SiH-- and (CH.sub.3).sub.3 Si--, but any one or more of R.sub.5, R.sub.6, R.sub.7 and R.sub.8 may be absent, depending on the valence of M and the unit structure.
(v) Compounds that contain any one or more of unit structures (i)-(iv) as partial unit structures in their chain or three-dimensional construction; or the mixture of such compounds.
The average molecular weight of the organic silicon polymer compounds employed as the starting material of the present invention is in the range of 800-20000 and advantageously in the range of 1000-5000 and has a slightly larger range than the polymers for spinning. It is preferable to remove preliminarily organic silicon polymers having an average molecular weight above 20000 because of difficulty in fusion and sheet manufacture. It is also advisable to remove as much as possible organic silicon polymers having an average molecular weight below 800 and contaminating compounds having a low boiling point, because the presence of such compounds may cause some troubles in anti-fusion treatment, flaking and sintering, namely perforation, sticking and irregular thickness of the final products respectively.
The average molecular weight of the organic silicon polymer according to the present invention (M=.SIGMA.MiNi/.SIGMA.Ni) is determined in tetrahydrofuran at 20.degree. C. by osmometry using a vapor pressure osmometer.
The fusion and softening point of these polycarbosilicon compounds varies depending on the distribution curve of the molecular weights and usually is in the range of 100.degree. C. to 350.degree. C. When the organic silicon polymers in block, gel or powder state are heated directly in the atmosphere of a non-oxidative gas, they change to the liquid of low viscosity at a temperature of 100.degree. C. to 350.degree. C.
The organic silicon polymers molded into plate and thin membrane-like sheets silicon polymers is also easily liquefied, deformed or gelled upon heating in the atmosphere of a non-oxidative gas at a temperature of 100.degree. C. to 350.degree. C., giving rise to inseparable products. In the method according to the present invention, the organic silicon polymer molded in a thin sheet is preliminarily subjected to heat treatment at a temperature of 50.degree. C. to 400.degree. C. in the atmosphere of an oxidative gas such as the air, oxygen and ozone (anti-fusion treatment) so that the initial shape of the product may be maintained during the subsequent heat treatment in the atmosphere of a non-oxidative gas.
When the air is used as the oxidative gas which is relatively mild and easily controllable, the temperature is slowly raised to 70.degree. C. during a period of more than 20 minutes, and preferably of 40-100 minutes, and the anti-fusion treatment is carried out at a temperature of 70.degree. to 400.degree. C., and preferably of 120.degree. to 240.degree. C., for a period of 30 minutes to 5 hours, and preferably of 1 hour to 3 hours. This anti-fusion treatment gives a uniformly thick and least wavy sheet, which assures no troubles such as shrinking, deformation, irregular thickness and perforation of the final product in the subsequent heat treatment.
It should be remembered, however, that a thickness of a sheet above 100.mu. before anti-fusion treatment often leads to irregular thickness or wavy surface on the subsequent heat treatment. Thus the upper limit of thickness of a sheet should be 100.mu.. In addition, when a sheet is less than 10.mu. thick before anti-fusion treatment, the handling thereof becomes extremely difficult and the occurrence of laceration and perforation during the anti-fusion treatment is clearly confirmed under a microscope.
Therefore the present inventors have defined the average thickness of the thin sheet for production of flaky materials to between 10.mu. to 100.mu..
Before describing the heat treatment of an infusible sheet specifically, it seems useful to explain some general scientific observations on the change of the organic silicon polymer sheet which is provided with the anti-fusion treatment. When an infusible organic silicon polymer sheet is heated to a high temperature above its melting point in the atmosphere of a non-oxidative gas such as N.sub.2, H.sub.2, NH.sub.3, Ar and CO gas, R.sub.1 -R.sub.8 in the aforementioned unit structures of the organic silicon polymer compounds (a hydrogen atom, alkyl, aryl, (CH.sub.3).sub.2 CH--, (C.sub.6 H.sub.5)SiH-- and (CH.sub.3).sub.3 Si--) begin to escape as volatile breakdown products around 300.degree. C., while the skeletal carbon and silicon components become amorphous, and the formation of .beta.-SiC starts at a temperature around 800.degree. C.
At this stage, several to several hundreds molecules of .beta.-SiC are formed without regular crystal lattice from the amorphous material mainly composed of Si and C. In other words, several to several hundreds molecules of .beta.-SiC are present in the dispersed state in the carbon-rich amorphous material mainly composed of Si and C.
As the temperature rises above 1000.degree. C. and subsequently above 1200.degree. C., the production of .beta.-SiC from the amorphous phase rapidly increases and consequently the percentage of the amorphous phase decreases, while the carbon excess advances in the amorphous phase.
The amorphous phase of the Si-C system obtained after heat treatment to a temperature below 1000.degree. C. is still labile and unfavorable for subsequent processing. As the formation of .beta.-SiC is relatively abundant and the activity of the amorphous phase diminishes at a temperature above 1200.degree. C., it is possible to handle the sheet as the stable flaky material even in the presence of oxygen.
Upon heating to a temperature above 1500.degree. C. in the atmosphere of a non-oxidative gas, mainly .beta.-SiC and carbon are produced.
The thermal treatment at a temperature above 1800.degree. C. is not satisfying, because the flaky material becomes fragile and loses mechanical strength.
Based on these observations, the present inventors have determined the range of the heating temperature for synthesis of the flaky products between 1200.degree. C. to 1800.degree. C.
The above describes the fundamental findings on the production of flaky .beta.-SiC from the organic silicon polymer and its preparation method.
In the following, several processes are presented for preparation of .beta.-SiC.
By methods detailed in the embodiments described later or other methods, a thin organic silicon polymer sheet having the thickness of 10.mu. to 100.mu. is prepared and is subsequently made infusible by known processes such as ozone treatment, heating in the air, .gamma.-ray irradiation or organic peroxide treatment. The obtained infusible sheet of the organic silicon polymer can be cut at this stage into small flaky pieces, each flaky piece having the length and the breadth 10-100 times greater than the thickness thereof, since the sheet has a sufficient mechanical strength to be handled without trouble. Then the thin sheet or the flaky pieces made of the infusible organic silicon polymer is heated in the atmosphere of a non-oxidative gas such as N.sub.2, H.sub.2, NH.sub.3,CO and Ar at a temperature of 1200.degree.-1800.degree. C. whereby a tenacious and elastic sheet or flaky piece mainly composed of .beta.-SiC is produced. The .beta.-SiC sheet is cut into flaky pieces having a regular or irregular shape depending on the applications or purpose of the use, and each piece has the length and the breadth 10-100 times greater than its thickness. As described above, flaking may be effected either before or after the heat treatment in the atmosphere of a non-oxidative gas.
According to the methods described above, flaky .beta.-SiC can be produced from the organic silicon polymer. The details of such methods are described in Example 1 and Example 2.
Subsequently, the properties of the microstructure of ceramics produced with flaky .beta.-SiC of the present invention will be explained.
First of all, the technique of the present invention to produce ceramics, and more particularly ceramics of laminar construction with .beta.-SiC is completely novel and has never been described in the literature.
Therefore, it will be useful to explain some special backgrounds of the ceramic industry which has clamored for the advent of such new technique with the new material from the viewpoint of thermal load. Thereafter, applications and features of ceramics according to the present invention are described.
The environments of thermal load applied on ceramics of the present invention are classified into four categories.
(A) When heat is suddenly supplied on one surface of a ceramic plate or on one inner or outer surface of a ceramic tube, the thermal stress exceeds the rupture strength of the ceramic, resulting in fracture. In this case, the source of fracture is located on the heated surface, because a fracture occurs in the restricted area neighboring the heated surface where the sudden thermal expansion of the heated surface is suppressed in vain.
(B) When heat is supplied from one surface of a ceramic plate or from one inner or outer surface of a ceramic tube, the thermal stress occurs by temperature gradient due to compression and tension. In this case, a fracture occurs on the unheated surface or the tension surface which has been known to be a weak point of ceramics.
(C) When a large ceramic plate or a long tube is exposed to heating, a concentric and radial fracture is observed.
(D) When a small temperature change is repeatedly produced locally on one surface of a ceramic plate or on one inner or outer surface of a ceramic tube, minute cracks develop by accumulation of thermal energy in the hysteresis loop even if the change is under the elastic limit, finally resulting in a fracture due to thermal fatigue.
As explained above, the environments of thermal load leading to the fracture of ceramics by thermal stresses are summarized as follows: (A) sudden and vigorous temperature change; (B) distortion by temperature gradient; (C) local heating; (D) thermal fatigue; and these load factors synergistically interact with each other in a complicated manner.
Under these circumstances, traditional ceramics have no physical strength sufficient enough to restrict their own expansion and are condemned not to be able to utilize plastic deformation like metals at a low temperature. Metals can effect enough plastic deformation by dislocation thereof, so that they may escape from fracture. Thus it has been one of the important tasks in ceramics to expand the breaking limits due to thermal stresses by using some structural control.
For promoting the dispersion of stresses, grains (grain boundaries), short fibers and long fibers have successfully been utilized in composite materials such as rubbers (FRR), plastics (FRP), metals (FRM) and concretes (FRC). In the ceramic industry, however, the application of thin sheets or flaky materials (or flakes) for dispersion of thermal stress has never been reported and thus might be one of the fruitful future approaches.
Flaky graphite is a sole hitherto known example of such flaky or fish-scale-like materials wherein the thermal stresses are dispersed by providing anisotropy to the structure of heat-resistant ceramics. However, graphite has a restricted range of applications, because it is least resistant to oxidation.
Although several papers are found in the literature which report that a heat-resisting woven cloth of boron fibers and alumina fibers adhered with resin is laminated as FRC, they completely differ from the present invention in applications, methodology and starting materials.
The term "laminar or stratified structure" used in the present invention is based on the fact that the resistance to thermal shock of a graphite crucible can be markedly improved by arranging flaky graphite in a concentric manner, and thus is defined as a structure where flaky material are arranged in a same direction. Such anisotropic structure permits the flaky materials and the interspaces therebetween to respond flexibly to thermal stresses.
The present inventors have succeeded in preparation of the ceramic sintered compact having improved resistance to thermal shock and thermal fatigue by forming a laminar structure made of flaky .beta.-SiC and producing appropriate spacings and bondings in the boundary area between such flaky materials as well as between the flaky material and the matrix by means of lamination with flaky .beta.-SiC. The laminar ceramics of flaky .beta.-SiC according to the present invention will be applied as the new material in the following fields:
(1) High temperature structural materials of the non-oxide system: Material: SiC, Si.sub.3 N.sub.4, SiC-Si.sub.3 N.sub.4 composite system, etc. Application: PA0 (2) Improvement of traditional ceramics containing graphite as the major or subsidiary material. PA0 Material: all refractory materials PA0 Application: refractory articles and crucibles
(a) Material for efficiency improvement and energy saving of furnaces. PA1 (b) Material for sophisticated ceramic parts PA1 (c) Material for other uses PA1 (a) Nozzles for pouring molten metals PA1 (b) Other refractory articles
Examples: PA2 Examples: PA2 Examples: PA2 Examples: PA2 Examples:
ceramic recuperator tube, PA3 ceramic radiant tube, PA3 ceramic duct for high temperature drafting, PA3 high-efficiency ceramic burner, etc. PA3 ceramic turbine blade, PA3 ceramic engine, PA3 ceramic nose cone PA3 high temperature anti-friction material, PA3 ceramic high-temperature coating, etc. PA3 continuous casting nozzle, PA3 immersion nozzle, PA3 long nozzle, PA3 new ladle nozzle, PA3 plates and nozzles for sliding-nozzle-type flow regulating device, PA3 stopper head, PA3 long stopper, etc. PA3 refractory inner lining for a blast furnace, PA3 trough material for a blast furnace, PA3 oxygen lance, PA3 thermal shock-resistant furnace part, PA3 graphite crucible, etc.
The shapes of ceramic articles listed above are mostly of hollow tube and are used under rigorous conditions of thermal load such as inner heating with outer cooling and vice versa.
In the high-temperature structural materials of the non-oxide system (application 1), ceramics may open a new field of research by substituting heat-resisting alloys. As is apparent in the attached examples, the laminar articles of flaky .beta.-SiC according to the present invention have shown markedly improved resistance to thermal shock and thermal fatigue in comparison with commercially available non-oxide sintered compacts.
On lamination or stratification of flaky .beta.-SiC, molding and synthesis (firing) were carried out well considering following factors;
(a) the orientation and flexibility (flexural strength) of the flaky materials,
(b) the absorption of expansion of the flaky materials,
(c) the relation between the mechanical strength and the bonding strength of the flaky materials, and
(d) the relation of the chemical bonding with the physical entanglement,
so as to cope with abrupt temperature change and thermal fatigue.
The ceramics of flaky .beta.-SiC of the present invention find their best utility in application (1) of high-temperature structural materials with high added values and express their advantages mainly in cylindrical or tubular shapes.
In application (2) of refractory articles, substitution of flaky graphite with .beta.-SiC in traditional refractory articles containing flaky graphite led to the surprising improvement of resistance to oxidation and thermal shock. In some cases of substitution of graphite, flaky .beta.-SiC did not show a laminar structure, but was oriented in random directions. Even under such random orientation, flaky .beta.-SiC, as flaky materials, retains to a certain extent a lamination-like structure disposing between coarse particles thus buffering thermal stresses. In addition, flaky .beta.-SiC per se can disperse thermal stresses. Compared with flaky graphite, flaky .beta.-SiC has a higher strength, a greater chemical reactivity and a larger friction in its surface, all of which contribute to strengthen resistance to thermal shock. In addition, flaky .beta.-SiC has a far better resistance to oxidation than flaky graphite. When used in refractories, .beta.-SiC on the surface or in the superfacial layer of articles is oxidized to Si0.sub.2, resulting in the volume expansion. It also reacts with other refractory components to form an anti-oxidation layer which prevents the drastic inward oxidation of the refractory articles.
By controlling the grain size of refractory components and the molding method, flaky materials in the refractory composition can be oriented in a laminar direction or in random directions.
Table 1 summarizes the orientation and the content of flaky graphite in commercially available refractory graphite articles together with molding methods.
TABLE 1 __________________________________________________________________________ Graphite Classification of Application or name content graphite depending (example) of (weight on the orientation refractories percentage) Molding method __________________________________________________________________________ Refractory stopper head 10-30 auger machine; articles in which uni-directional molding flaky materials (repress) form seemingly graphite crucible 20-60 auto-spinning method; laminar or uni-directional molding stratified graphite lance 10-60 vacuum extrusion molding structure graphite nozzle 15-35 vacuum extrusion molding sliding nozzle plate 1-15 uni-directional molding graphite brick for 20-65 uni-directional molding a blast furnace Refractory graphite nozzle 15-40 isostatic press molding articles in which graphite material 10-30 flow molding or vibration flaky for a trough molding materials or a graphite crucible 20-90 isostatic press molding group of flaky materials show random orienta- tion __________________________________________________________________________
Embodiments 6 and 7 disclose refractory articles with improved resistance to oxidation and thermal shock which are manufactured by substituting a partial or the total amount of flaky graphite in the refractory articles listed in Table 1 with flaky .beta.-SiC of the present invention.
Embodiment 8 discloses a refractory brick with new features which is obtained by admixing flaky .beta.-SiC of the present invention. Some graphite articles in which .beta.-SiC can not replace flaky graphite; for example, basic refractory inner linings which are used for refining molten metals at a high temperature above 1600.degree. C. in the presence of basic slug (MgO-C system, CaO-C system and MgO CaO-C system) are not included in Table 1. Excluding such exceptions, the combined use of, or the replacement with, flaky .beta.-SiC generally results in the improvement of resistance to oxidation and thermal shock of refractory graphite articles. The amount of .beta.-SiC varies depending on the applications but usually in the range of 1 percent to 60 percent by weight.
Hereinafter, the method of the present invention for producing flaky .beta.-SiC, their applications and their advantages will be discussed in view of following embodiments, but such embodiments are solely for explanation purpose and should not be construed to limit the scope of the invention in any way.